The present invention relates to a process of preparing alumina useful as a catalyst carrier, and in more detail relates to a process of preparing alumina designed so that in the course of kneading an amorphous alumina hydrate cake produced by the neutralization reaction of aluminum sulfate with sodium aluminate, the degree of growth of pseudoboehmite grains contained in said cake may be controlled. The gamma alumina or eta alumina obtained by drying and calcining a pseudoboehmite gel-containing amorphous alumina hydrate has hitherto been widely utilized as a catalyst carrier because said alumina has a large specific surface area and is also superior in thermal stability. As is generally known, the performance of a catalyst supported on alumina owes much to the physical and chemical properties of the alumina used in the carrier, in particular its physical properties, namely the specific surface area and porous characteristic, because these are important factors which control the performance of a catalyst supported on alumina. However, as the specific surface area and pore characteristic required for alumina carrier vary depending on the particular catalysts for which said alumina is used, the specific surface area and pore characteristic called for in the alumina carrier are not uniform.
The greatest importance of the alumina carrier used for a hydrocarbon reforming catalyst, for instance, consists in the fact that said alumina carrier has a large specific surface area, rather than the nature of its pore characteristic. In contrast, in the case of the alumina carrier used for a desulfurizing catalyst, it is important that said carrier has a pore characteristic suitable for the molecule taking part in the desulfurization reaction, more specifically, that the carrier has an average pore diameter a pore volume suitable for the desulfurization reaction. Referring to the specific surface area in this instance, although the catalytic activity also increases as the specific surface area increases, the pores of less than 50 .ANG., which are very remarkably attributable to increase in specific surface area, are easily plugged by coke deposit. In order that the catalyst may hold its stability even if the specific surface area somewhat deteriorates, it is rather preferable that micro-pores of less than 50 .ANG. should not exist. On the other hand, an alumina carrier containing little impurities is desirable, because impurities present in a catalyst carrier exert an enormous influence upon the chemical properties depending on its usage.
In these circumstances, when preparing an alumina carrier from the amorphous alumina hydrate, there has usually been employed the process which comprises stirring an aqueous slurry of said amorphous alumina hydrate at elevated temperatures and under weak alkaline conditions to thereby control the degree of growth of pseudoboehmite grains contained in said slurry, thereby regulating, as requested, the pore characteristic and specific surface area of the finally obtained alumina carrier. However, the aforesaid prior art process is not always advantageous in that it takes a relatively long period of time to make the pseudoboehmite grains grow in the aqueous slurry of the amorphous alumina hydrate.